1. Field of the Invention
The present invention relates to an antenna structure, and more particularly, to a folded multi-band antenna capable of improving impedance matching and adjusting its operating frequency bands.
2. Description of the Prior Art
As wireless telecommunication develops with the trend of micro-sized mobile communication products, the location and the space arranged for antennas are limited. Therefore, some built-in micro antennas have been developed. Currently, micro antennas such as chip antennas, planar antennas etc are commonly used. All these antennas have the feature of small volume. Additionally, planar antennas are also designed in many types such as microstrip antennas, printed antennas and planar inverted F antennas (PIFA). These antennas are widespread applied to GSM, DCS, UMTS, WLAN, Bluetooth, etc.
Please refer to FIG. 1. FIG. 1 is a diagram of a conventional planar inverted F antenna (PIFA) 100 according to the prior art. The PIFA 100 consists of a radiation element 110, a grounding element 120, and two conductive pins 130 and 140. The conductive pin 130 is coupled to the grounding element 120 to be used as a grounding point, and the conductive pin 140 passes through the grounding element 120 and is further coupled to a wireless transceiver circuit (not shown) to be used as a signal feeding point. In this way, when the conductive pin 140 feeds a current into the radiation element 110, the current is divided into two current paths I1 and I2. Path lengths of these two current paths I1 and I2 are different from each other, wherein the path length of the first current path I1 is approximately one-fourth of a wavelength (λ/4) of a first resonance mode generated by the planar inverted F antenna 100 and the path length of the second current path I2 is approximately one-fourth of a wavelength of a second resonance mode generated by the planar inverted F antenna 100. In other words, the conventional PIFA 100 is capable of transmitting/receiving electromagnetic waves of two different frequencies.
Since the radiation element 110 of the conventional PIFA 100 is a rectangular-shaped plane, it occupies a large area, which is inconsistent with market demands of thin and light volume. In addition, as the conductive pins 130 and 140 are disposed between the radiation element 110 and the grounding element 120, its size and location are fixed. Accordingly, it is difficult to adjust impedance matching and operating frequency band of the conventional PIFA 100 depending on design requirements.